Field of the Invention
The present invention relates to a polyester film and a method for producing the same, a back sheet for a solar cell, and a solar cell module.
Background Art
A solar cell module generally has such a structure that contains glass or a front sheet on a light receiving surface, on which the solar light is incident, having laminated thereon in this order a transparent filling material (which may be hereinafter referred to as a sealant), a solar cell device, a sealant and a back sheet. Specifically, a solar cell device is generally embedded with a resin (sealant), such as EVA (ethylene-vinyl acetate copolymer), and a protective sheet for a solar cell is further adhered thereon. It has been known that a polyester film, particularly a polyethylene terephthalate (which may be hereinafter referred to as PET) film is used as the back sheet for a solar cell.
However, the protective sheet for a solar cell, particularly a back sheet for a solar cell used as the outermost layer, is assumed to be exposed outdoor weather for a prolonged period of time, and thus is demanded to have excellent weather resistance.
A polyester film, such as PET, used as a back sheet for a solar cell is excellent in heat resistance, mechanical characteristics, chemical resistance and the like, and thus is frequently used industrially, but still has room for improvement in hydrolysis resistance. It has been known that a polyester film may be deteriorated by hydrolysis with the lapse of a prolonged period of time.
It has been known that the hydrolysis is accelerated by the catalytic action of the terminal carboxylic acid in the polyester, and as a technique for improving the hydrolysis resistance of a polyester film as a measure thereto, it has been known to use a terminal blocking agent, such as a polycarbodiimide, capable of reacting with the terminal carboxylic acid (see, for example, Patent Literature 1). Patent Literature 1 describes that a polyester film using a terminal blocking agent, such as a polycarbodiimide, is controlled to have particular characteristics, thereby providing an electric insulating film that has excellent hydrolysis resistance and excellent durability.
Patent Literature 2 describes that a conductive layer and a readily adhesive layer are provided on a polyester film, such as PET, used as a back sheet for a solar cell, thereby imparting a high partial discharge voltage.